The flywheel has long been used to store energy. The potter's wheel, invented approximate 5,000 years ago, probably was one of the first devices to employ a flywheel for energy storage.
The flywheel is attractive from an energy storage perspective for a number of reasons. It is a relatively simple device from which it is possible to store and abstract energy readily, either by mechanical means or by using electric motors and generators. In theory, high power generation rates are possible and there are no inherent limitations on the number of charge and discharge cycles that can be used. Charge capacity should not be affected by age or the number of prior charge and discharge cycles, which is not the case for known batteries.
The energy density capability of a flywheel, also known as its specific energy, is given as: EQU T=K.sub.s .sigma./.rho.
where T is the specific energy, K.sub.s is a flywheel shape factor, .sigma. is the tensile strength of the material from which the flywheel is constructed, and .rho. is the material density. Energy density typically is expressed in watt-hours per pound (W-hrs/lb), watt-hours per kilogram (W-hrs/kg), or joules per kilogram (J/kg).
Flywheels may be generally characterized as isotropic or anisotropic depending on their material structure; flywheels combining certain features of isotropic and anisotropic forms are also known. Isotropic flywheels typically are designed such that each particle of the flywheel mass is stressed equally in all three orthogonal dimensions. In general, these flywheels are very thin at their circumference and thick at their hub. However, there are many possible variations which involve bulges or other shapes on the outer edge. These variations attempt to maximize the rotational inertia of the flywheel--and, hence, increase the flywheel's energy storage capability--by holding the entire flywheel mass at the same stress level while at the same time placing as much mass as possible at the outer portion of the radius. Unfortunately, the energy density is very poor for all known materials that appear useful in isotropic systems. Expensive tool steel probably provide the best example of materials which can be machined to the desired shapes. However, tool steels cannot be processed into large sizes without the potential for material flaws which lead to crack propagation during high stress loads.
Anisotropic flywheels typically utilize materials which possess exceedingly high strengths in one dimension but considerably lower strengths in the other dimensions. These devices usually have comprised wound fibers. Accordingly, most anisotropic flywheel possess only nominal radial strength but are able to withstand a great deal of tangential stress. However, because fibers typically are weak in the radial direction and because said flywheels have been axially wound, these configurations must usually be constructed as thin rims. A rim can carry the highest known energy density because it contains all of its useful, high inertial mass at the outermost radius. The only theoretically limiting factor is the maximum allowable tangential stress for the constituent material. Accordingly, known energy storage devices constructed with thin rims--while possessing a very high energy density--typically cannot store large amounts of energy.
For both isotropic and anisotropic flywheels, the key to storing large amounts of energy is to employ materials having high specific strength, that is high maximum tensile stress in relation to weight density. Many of the materials presently known in the art to have high specific strength are available only as fibers. These materials typically cannot be cast or machined into the shapes required for fabrication of isotropic flywheels. Isotropic flywheels presently can only be made from metallic materials which have relatively low specific strengths. Even if strong composite materials were to be developed which could be machined or cast into isotropic structures, they likely still would have the same problems of all rotational wheels which transfer their radial loads back to the rotational axis unless they were somehow de-coupled as in the form of concentric rings. Therefore, isotropic flywheels probably will always have an energy density about an order of magnitude lower than anisotropic, rim-type flywheels. Accordingly, anisotropic flywheels are preferred for energy storage applications.
However, many problems have been encountered in the design and construction of anisotropic flywheels which can be operated at the high RPM which are desired for energy storage. Significant improvements in anisotropic flywheels were made possible by the disclosures of R. F. Post in U.S. Pat. No. 3,683,216 and of R. F. Post and S. F. Post, Scientific American, 1973, 229;6, 17-23, suggesting the use of fiber composites to increase energy density. The Posts proposed that a set of fiber wound rims of decreasing outer radial material weight densities would meet the conditions for maximizing the rotational inertia --and, hence, kinetic energy storage--while still decoupling the very high radial stress loads that would ensue if the material was made to be contiguous from axis to outer periphery.
Theoretically, the Post design can achieve both high energy densities and high volumetric energy storage potentials. However, this design has a number technical problems relating to the manner in which the rim or rims are connected to the hub. In order to obtain a system that can effectively withstand dynamic and environmental vibrations, the rim must be rigidly supported about its rotational axis as it changes shape during operation and rotation speed variation. A considerable amount of research over the last two decades has been directed toward the development of anisotropic flywheels which are dynamically stable from rim to hub.
For example, T. W. Place, "Composite Material Flywheel for UMTA Flywheel Trolley Coach", 1980 Flywheel Technology Symposium, October 1980, Scottsdale, Ariz., disclosed rim-type rotors comprising a few tightly fit fiber rims of decreasing density from the inside diameter outward. The rims were wound in compression on each other and press warped in a non-circular cruciform spoke configuration. This shape placed non-uniform stresses on the wound material, suffered from high stress loads at the spoke contacts, and experienced high point load strains on the wound material due to nonuniform radial growths which necessarily occur because of the compressive spoke contact loadings. This configuration was also not successful because of unsymmetrical warping about the axis of rotation, which prevented the wheel from remaining in balance at all speeds from zero to its maximum design value.
P. C. Poubeau, "Flywheel Energy Storage Systems Operating on Magnetic Bearings", 1980 Flywheel Technology Symposium, October 1980, Scottsdale, Ariz., disclosed flywheels wherein material is wound around radial spokes. Instead of winding the rim with composite fibers, these flywheels employ very high strength piano wire. However, even the highest grades of steel wire still have considerably lower specific strengths than composite materials. Therefore, the potential for high energy density storage is more limited even than the concept suggested by Place.
The flywheels proposed by both Place and Poubeau satisfy the need for high inertial volumetric packing; that is, they place the majority of the flywheel weight at the outer periphery. The flywheels are also able to radially decouple from the hub assembly. However, these designs produce high point stress loads on the wound rim and produce an undesirable concentration of bending strains which unduly limit the total energy storage by failure at the spoke locations. They are also subject to high dynamic imbalance problems due to the wide, variable stretch which can lead to non uniform differential radial strains between spokes.
S. F. Post and F. C. Younger, "Design and Fabrication of a Flywheel Rotor for Automatic Use", 1980 Flywheel Technology Symposium, October, 1980, Scottsdale, Ariz., proposed a configuration which employs rims of differing weight density. The rims are radially detached from the hub assembly, thus eliminating the coupling of high radial stresses down to the axis of rotation. This design uses a set of composite wraps attached to the rim by weights. During periods of relatively high RPM, the banded wraps (called tensioned balanced catenary spokes by the authors) press down on the hub assembly, creating a type of coupling between the rim and hub. However, the configuration requires a multitude of wraps to attain the desired bearing pressure loading on the hub and, consequently, cannot be practically manufactured. Also, this configuration fails to offer good, uniform axial stability between the rim and hub, largely because it cannot arrest slight axial rim vibrations. Slight off-axis centering between the rim and hub can occur during normal vibration, thus creating imbalance in a highly dynamic system. Finally, the weights attached to the inside surface of the composite wraps put a great deal of pressure on the wrap itself, thereby creating a point of weak contact.
D. G. Ullman and J. Corey, "The Accelerating Flywheel", 1980 Flywheel Technology Symposium, October, 1980, Scottsdale, Ariz., proposed a flexible flywheel rim configuration which attempts to decouple the radial stresses from the rim to hub. However, there are at least two major difficulties with this flywheel design. First, the spring constants between the flywheel's hub and rim are very small, creating a situation where adverse radial micro-movement of the rim system cannot be arrested by the hub's radial loading about the axis of rotation. Accordingly, the flywheel is highly subject to dynamic instabilities with any modal oscillation. Secondly, assuming that the flywheel could get up to speed without undergoing radial oscillation, a serious problem would occur when power is extracted from the flywheel. The rim would continue rotating because of its own high inertia, while the loose bands which connect the hub to the rim would instantly unwind or tend to reverse with high peak tension loads because there is no azimuthal rigidity between these structures.
C. E. Knight, "Analysis of the Deltawrap Flywheel Design", 1977 Flywheel Technology Symposium Proceedings, October 1977, San Francisco, Calif., disclosed a configuration wherein rims or discs are mounted to hubs and an overwrap material surrounds the entire device. The rationale behind this flywheel is that the overwrap holds the flywheel stationary about its axis of rotation and provides additional strength in the radial direction. However, the enhanced radial strength comes at the expense of additional weight from the overwrap, which must be constructed with thick fiber bands of uniform density in order to maintain rigidity of the entire rotating structure. Also, small radial growths will naturally occur in a non-uniformly azimuthal fashion, since the expansion of the overwrap cannot allow for precisely equal radial growths over the entire azimuth. This then produces a condition of highly variable dynamic imbalance.
S. V. Kulkarni, "The Flywheel Rotor and Containment Technology Development Program of the U.S. Department of Energy", 1980 Flywheel Technology Symposium, October, 1980, Scottsdale, Ariz., investigated a design which employed a thick, multi-layered, composite disc in which the fibers in each layer are parallel to one another, but each layer is slightly rotated relative to the next. The entire set of discs is bonded or laminated into one thick disc. A parallel fiber configuration maintains each layer in a condition where along one axis the fibers are in tension while in the orthogonal direction there is virtually no fiber strength. The entire strength comes from the epoxy used to bind the fibers, which greatly limits the strength potential of the fiber and limits the attainable energy density to relatively lower values. In these designs, higher strength composite fiber materials were chosen rather than isotropic metal fibers in order to take advantage of the greater strength potential of the fibers. The materials employed met the condition of high strength, but the design configurations were limited in optimal stress management and weight distribution. Therefore, the flywheels could not attain as high an energy density as rim type designs.
Thus, despite the intense efforts of those in the art, there still exists a need for practical flywheels which are useful in energy storage applications.